The prior art is replete with various devices for ensuring that radio frequency or electromagnetic waves do not penetrate into or out of a sealed enclosure. Early prior art devices were designed to try to protect highly sensitive computers by preventing radio frequency and electromagnetic waves from entering a sealed enclosure containing the computer equipment. Due to the classified nature of the programs which various computers utilize, and the use to which they are directed, later patents directed to shielding a particular environment were drawn to various methods and devices for preventing electromagnetic or radio frequency waves from being transmitted from the shielded enclosure to the outside environment. these devices were important because highly sophisticated methods were being developed to eavesdrop on highly confidential governmental communications and operations.
Recently, a new problem has presented itself in the form of an extremely high frequency electromagnetic pulse which is produced by the detonation of a nuclear blast high in the atmosphere. This problem is documented in the January/February 1983 issue of Science magazine (pp. 41-49) wherein the deleterious effects of this electromagnetic pulse are described with respect to the problems which are imposed upon communications ystems and shielded enclosures. These high frequency electromagnetic waves could play havoc with the classified material contained in computers which is to be protected in the enclosed environment.
The majority of prior art devices, which have dealt with ensuring that radio frequency or electromagnetic waves do not penetrate into or exit from the sealed environment, generally protect the environment from relatively low frequencies of radiation, but does not afford comparable attenuation levels at microwave frequencies. As the computational speed of modern digital computers has increased so has the frequency content of electromagnetic emanations from these computers and digital communications systems. Therefore, a door seal that is more effective at microwave frequencies is required to protect the classified material being processed within a shielded enclosure.
It has been established that the voltage drop across the seal on the inside surface of all shielded enclosure or equipment access doors is an important consideration when designing a seal which would protect a shielded enclosure from the egress of very high frequency electromagnetic waves through the shielded door. In this context, all of these doors require exremely good metal-to-metal contact around their periphery.
Prior art patents which have dealt with this problem of providing an adequate door seal employ beryllium copper or phosphor bronze spring fingers within the door seal. Such prior art door seals are described in U.S. Pat. Nos. 3,589,070 issued to Hansen and 4,069,618 issued to Geiss. Both of these patents describe door seals used is shielded enclosures. These patents employ a knife edge provided on a first closure member which cooperates with spring fingers provided on a second closure member to form a door seal for screen rooms and shielded enclosures. While it has been shown that the door seal configurations shown in the Hansen and Geiss patents are quite effective in forming a good closure at low frequencies, at very high frequencies in excess of 0.5 GHz, the inductance of the tines of the spring fingers increases and the leakage through the slots formed in the door seal increases, thereby decreasing the effectiveness of the door seal.
The patent to Geiss also uses a woven metallic RF gasket in the area of cooperation between the knife edge and the spring fingers. This gasket introduces another low frequency current path in parallel with the two rows of spring fingers thereby reducing the voltage drop across the door seal. Secondly, the presence of this gasket increases the microwave attenuation of the door seal by acting as another barrier to the microwave energy. This material does not have the resilience of the beryllium copper spring fingers and ultimately it will "take a set" and added attenuation of the waves will be lost.
While different RF absorbing materials can be used between the rows of spring fingers to improve the microwave attenuation of the dor seal, the limited space between the spring fingers of approximately one centimeter limits the effectiveness of the door seal.
Consequently, it is seen that there is a continuing need for a more effective door seal which would block both low frequency electromagnetic waves as well as very high frequency electromagnetic waves.